(1) Field of the Invention
The present invention relates generally to the manufacture of annealed ferroelectric copolymer materials, and more particularly to a method for increasing fracture toughness and reducing brittleness of a semi-crystalline ferroelectric polymer material such as poly(vinylidene fluoride-trifluorethylene) or p(VDF-TrFE).
(2) Description of the Prior Art
Many semi-crystalline polymers become brittle when formed into thin sheets. In terms of a quantitative measure, these materials have a low fracture toughness which is measured as energy per unit volume in Joules/meter3 (J/m3). Brittleness is caused by a high percentage of crystallinity and/or an increased average size of the polymer crystallites brought about by the manufacturing process. As a result of the material""s brittleness, damage during normal handling thereof is prevalent thereby increasing the cost of using semi-crystalline polymers in various products.
In some applications, crystallinity percentages in excess of 80% are desired or required in order for the semi-crystalline material to perform properly. For example, the use of ferroelectric p(VDF-TrFE) has been problematic because it is necessary to anneal the material to a very high level of crystallinity in order to maximize the material""s piezoelectric properties. However, while the annealing step greatly increases the material""s crystallinity in preparation for a ferroelectric poling operation, this processing step also makes the treated material so brittle that it often cracks during routine handling thereof.
Accordingly, it is an object of the present invention is to provide a method for increasing a semi-crystalline ferroelectric material""s fracture toughness to thereby reduce its brittleness without substantially damaging the material""s ferroelectric properties.
Another object of the present invention is to provide a method to increase the fracture toughness of an annealed ferroelectric polymer material while substantially maintaining the material""s ferroelectric properties.
A still further object of the present invention is to provide for increased use of ferroelectric p(VDF-TrFE) in applications where the material""s brittleness previously prevented such use.
Still another object of the present invention is to reduce the brittleness of ferroelectric p(VDF-TrFE) while retaining its ferroelectric properties.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is provided that increases fracture toughness and reduces brittleness of a semi-crystalline ferroelectric polymer material while substantially maintaining ferroelectric properties of the material. A semi-crystalline ferroelectric polymer material such as poly(vinylidene fluoride-trifluorethylene) is placed in an inert oxygen-free atmosphere. The material is heated to a temperature that is greater than the material""s Curie transition temperature, but below its melting temperature. The material is then irradiated with beta particles to provide a desired level of fracture toughness that substantially maintains ferroelectric properties of the material. In the case of poly(vinylidene fluoride-trifluorethylene), the heating temperature is approximately between 100-120xc2x0 C., the beta particles have an energy level of approximately 2.5 mega electron volts (MeV), and the radiation dose does not exceed approximately 50 megarads (Mrads).